There are known two large groups of solid-state radiation detectors, which dominate the area of ionizing radiation measurements, scintillation detectors and semiconductor diodes. The scintillators detect high-energy radiation through generation of light which is subsequently registered by a photo-detector that converts light into an electrical signal. Semiconductor diodes employ reverse biased p-n junctions where the absorbed radiation creates electrons and holes, which are separated by the junction field thereby producing a direct electrical response.
Many scintillators are implemented in wide-gap insulating materials doped (“activated”) with radiation centers. One of the major benefits of using semiconductor materials is the mature state of technology that enables the implementation of epitaxial photodiodes integrated on the surface of a semiconductor slab.
FIG. 1 shows a cross-section of the prior-art semiconductor (InP) scintillator with an epitaxially integrated photodiode created on one of its sides or surfaces.
The prior-art pixel architecture in the form of the cleaved pixel design is illustrated in FIG. 2. The key motivation for this design is as follows. One is concerned with ensuring spatial independence of the scintillation flux on the pixel photodiodes, so that the readout would be the same irrespective of where the interaction occurred in the volume of pixel. Due to the high refractive index of InP, only a small fraction of radiation impinging on the side surface from a random point will escape, while most of the radiation will be internally reflected and eventually reach the absorbing photodiode. If the scintillator material is highly transparent to its own luminescence, the cleaved pixel design should deliver most of the radiation generated within the pixel to the photodiode on its surface. The ideal highly transparent semiconductor scintillator remains elusive and one of the objects of the present invention is to accommodate some degree of optical extinction in the scintillator material. Another problem of the integrated pixel design is the large capacitance of the pixel photodiode. FIG. 2 illustrates an example for the pin diode having thickness of about 2 μm and area of 1 mm×1 mm, where the capacitance is about 50 pF. The large diode capacitance makes it difficult to detect small amounts of photo-generated charge.
We are referring now to FIG. 3, which illustrates an alternative prior-art design that addresses the issue of high capacitance. The idea is to make an array of photodiodes of much smaller area than that of the detector itself, without cleaving. The full power of planar integrated technology is then deployed. For example, if 100 μm×100 μm diodes are used, instead of 1 mm×1 mm, the capacitance goes down by two orders of magnitude. However, the number of photons collected by a single mini-pixel decreases, albeit by a smaller amount. The reason for the decreasing number of photons is because photons generated by a single ionization event (a Compton or photoelectric interaction in the given slab) are now shared by several 2D photodiodes. The total area of illuminated photodiodes is of the order z2, where z is the distance from the interaction region to the top surface of the detector, i.e. the surface where the epitaxial photodiodes are located in the prior art design. Inasmuch as z2<<1 mm2, the amount of photons received by a nearest single 100 μm×100 μm photodiode decreases by the smaller amount, compared to the 100-fold decrease in the area. The integrated pixel design thus enhances the charge per capacitance ratio. This implies a higher voltage developed in a single diode in response to receiving the scintillating radiation and therefore a higher signal to noise ratio.
One of the advantages of the integrated pixel design associated with the present invention is the capability of assessing the position of the ionizing interaction (i.e., the distance z from the interaction region to the top area of the detector) from the measured lateral response distribution of the 2D photodiode array. Such assessment cannot be expected to be very accurate, but one can indeed expect that an ionizing event at the distance z deep into the scintillator slab will illuminate a circular spot on the surface of radius r≈z with the illumination intensity decreasing radially away from the nearest photodiode.
The problem associated with the prior art designs of FIG. 2 and FIG. 3 relates to insufficient transparency of InP at room temperature to its own scintillation. This problem is illustrated in FIGS. 4 and 5, which show the luminescence spectra originating from the photoexcitation near one of the InP slab surfaces and observed either from the side of incident excitation or from the opposite side of semiconductor slab. The former spectrum is referred to as the reflection luminescence and the latter the transmission luminescence. FIG. 4 is a schematic diagram illustrating the geometry of the photoluminescence experimental arrangement with InP wafer which is shown as a slant member with the light being directed to the wafer. Some of the light is reflected back, as a reflection luminescence. The spectrum of the reflection luminescence is measured by a measuring device which can be in the form of monochromator. Some of the luminescence goes through the entire wafer and is collected by another measuring device or monochromator on the other side thereof, in the form of transmission luminescence. For a transparent material a transmission luminescence will be as strong as a reflection luminescence.
FIG. 5 shows the spectra obtained at room temperature (300K). Luminescence spectra of 350 μm thick InP wafer doped with shallow donors (S) to the level ND=6.3×1018 cm−3, addition to the reflection and the transmission luminescence spectra, FIG. 5 shows the spectrum of transmission of incident light across the wafer.
As can be seen in FIG. 5, substantially the long-wavelength portion of the reflection luminescence spectrum is observed in the transmission geometry and the optical power of the transmission luminescence signal is at most 20% of the reflection luminescence signal. This constitutes a major problem for some of the intended applications of the scintillator, associated with the accurate determination of the deposited energy. The problem is how to distinguish the signal arising from a substantial energy deposited far from the photoreceiver surface from that arising from smaller energy deposited near this surface.
The problem is clearly owing to the attenuation of the optical signal. Still, if the distance z from the photoreceiver surface of the high-energy radiation absorption (ionization) event will be known, correction for the attenuation will be possible. For the prior-art scintillators, the distance z (see FIG. 3) is not accurately knowable. To some extent, one could deduce the distance z from the intensity distribution in the illuminated spot in the integrated photo-diode array architecture, as previously discussed in connection with the prior art design shown in FIG. 3. However, this approach has not been considered in the prior art, in part because it cannot be expected to provide an accurate estimate of the position and in part because it would place extremely high demands on the sensitivity of individual pixels.